You've heard it all before: Don't go swimming for an hour after you've eaten. Cross that bridge when you come to it. You'll be able to live on 70% to 80% of your current income in retirement.

Unfortunately, we're qualified to address only the last adage (though if you can cross a bridge before you've come to it, we'd like to see pictures).

The "70% to 80%" rule of thumb is based on the assumption that many of your current expenses will go away in the golden years. For the most part, this is true. Once you kiss the boss goodbye, you'll no longer endure the following:

Work-related expenses, such as commuting costs, professional wardrobe upkeep, and cubicle decor;

Social Security taxes (15.3% of income for the self-employed, 7.65% for the other-employed);

Contributions to retirement plans (it's time to stop the giving and start the taking);

Mortgage payments, if your house will be paid off by the time you retire.

There are two other ways your expenses might decline. First, retirees tend to downsize. There's no need to keep up the four-bedroom house now that the kids are grown, for example, so some pensioners sell the big house and move to the Sag-A-Lot colony for mature nudists. Secondly, overall taxes might decrease. Only a portion of Social Security benefits are taxable -- if at all -- and income might be derived from long-term capital gains, which are taxed more gently than ordinary income.

Some studies support the 70% to 80% rule. The 2001 RETIRE Project report, conducted by Georgia State University, found that retirees would need 74% to 83% of their income to maintain the same lifestyle in retirement.

However, some retirees still require the same level of income in retirement as they enjoyed while they were working. How? It comes down to this: If you're not making money, you're spending money. You have to fill the free time somehow. New hobbies, trips to Europe, and gas-guzzling RVs cost money. Many participants in our Rule Your Retirement seminar conducted last summer said they planned on spending less in retirement, only to find themselves shelling out even more.

What will retirement cost for you? There's only one way to find out: Create a retirement budget. To get an idea of how much you'll pay in taxes, complete a sample return -- if you use an online service or software to prepare your taxes, this won't be so daunting. The Fool can help you develop a plan, too. Check out our Rule Your Retirement Online Seminar, starting Nov. 14. Once you know how much retirement will cost, make sure you're saving enough to pay for the hobbies, trips, and nudists.